The overall objective of the project is to elucidate the reactions of antithrombin III with thrombin and other plasma procoagulants, and to define the mechanisms by which heparin inhibits blood coagulation. Since many of the reactions involved in coagulation are inhibited by antithrombin III and heparin, the data obtained from these investigations should aid in elucidating various processes that regulate and control blood clotting. One aspect of the research will be concerned with the determination of the structure of the highly active heparin previously isolated in this laboratory by gel-filtration and affinity chromatography on heparin-aminohexylsepharose. Specifically, it will be our aim to define the structural unit critical to anticoagulant action. Parallel experiments will be performed to determine the effect of modification of pivotal functional groups. The high-activity heparin fraction will also be utilized for studies of the interactions of heparin with antithrombin III and in the binding of the latter with plasma procoagulant proteins. Another area of study will be to determine various structural aspects of the complexes formed between thrombin and antithrombin III, and the effect of heparin on the structure and stability of the complex. Similar investigations will also be conducted on the binding of antithrombin III with factor Xa.